Hi, my first time here so please be gentle - I need some help!

I have added PayPal e-commerce to my website and I am trying to get the shopping cart/checkout to create total shipping costs based on the number of items ordered (where customer may buy one or more of several different items):

Example:
Product A: 3.50 for first item, then 2.50 for subsequent items
Product B: 2.50 for first and subsequent items (if buyer also orders product A) OTHERWISE 3.50 for first item, then 2.50 for subsequent items.
Product C: 8.00 for first item, then additional 4.00 for second and third items. Fourth and subsequent items at no additional charge.
etc.

Therefore...
If a customer buys 3 x Product A plus 2 x Product B and 4 x Product C, the total shipping should be 29.50

but...
PayPal sandbox tests give me a total of 34.50 (5.00 too much!)



Now that I've seen all that in writing, it looks very complicated!

Thanks for any help you can come up with.

Rod
PS - I have deliberately omitted the currency symbol above so as not to confuse matters even further with £/$ signs!